The present invention relates to a system and a method for improving performance of an exhaust gas aftertreatment device and, more particularly, to using an air assist heated reductant delivery system to improve NOx conversion efficiency and reduce fuel penalty.
Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices such as, for example, Active Lean NOx Catalysts (ALNC), which are capable of continuously reducing NOx emissions, even in an oxygen rich environment. In order to maximize NOx reduction in the ALNC, a hydrocarbon-based reductant, such as fuel (HC), has to be added to the exhaust gas entering the device. However, introducing fuel as a reductant reduces overall vehicle fuel economy. Therefore, in order to achieve high levels of NOx conversion in the ALNC while concurrently minimizing the fuel penalty, it is important to optimize usage of injected reductant.
In this regard, it is known that improved NOx conversion can be achieved by introducing the reductant in vapor rather than liquid form due to better distribution and mixing of the reductant with the exhaust gas entering the NOx reduction device.
One such system is described in U.S. Pat. No. 5,771,689, wherein a reductant is introduced into the exhaust gas via an evaporator device that has a hollow body with a heating element protruding into its interior. The evaporator device protrudes into the wall of the exhaust pipe upstream of the catalyst. The reductant is introduced so that it flows through the narrow space between the hollow body and the heating element until it reaches the tip of the heating element from where it enters the exhaust pipe in vapor form and mixes with the exhaust gas entering the catalyst.
The inventors herein have recognized several disadvantages with this approach. Namely, if delivery of the reductant has been shut off, or reduced, as dictated by the operating conditions, some reductant may remain in the annular space, in contact with the heating element, and may therefore clog up the opening around the heating device by carbonation of the residual fuel. Such carbon build up may lead to a blockage of the passage at the tip by which the vaporized fuel enters the exhaust stream. Further, there is a delay in introducing the reductant into the exhaust gas stream due to the time it takes for the reductant to travel down the length of the heating element. Additionally, durability of the heating element is reduced because its temperature is not controlled and adjusted based on operating conditions, and due to soot contamination. Yet another disadvantage of the prior art approach is that extra power is consumed due to the above-mentioned lack of temperature control.
The present invention teaches a system and a method for introducing evaporated reductant into an exhaust gas stream entering a lean exhaust gas aftertreatment device while eliminating the above-mentioned disadvantages of the prior art approaches.
In accordance with the present invention, a reductant delivery system includes: an evaporator unit including at least a heating element; a mixing device having at least one inlet and at least one outlet, said outlet coupled to said evaporator unit; and a controller for introducing reductant and air into said mixing device through said inlet, injecting a mixture of said reductant and said air through said outlet into said evaporator unit thereby causing evaporation of said reductant and air mixture.
In another aspect of the present invention, a method for controlling a reductant delivery system having at least a heating element, the system coupled upstream of an exhaust system of an internal combustion engine in a mobile vehicle, includes: injecting air into the reductant delivery system; injecting a reductant into the reductant delivery system thereby creating a vaporized mixture; and directing said vaporized mixture into the exhaust system of the engine.
The present invention provides a number of advantages. In particular, creating a mixture of reductant and air improves the exhaust gas aftertreatment device efficiency, due to the enhanced mixing of the reductant with the bulk exhaust flow and improved catalytic action relative to the use of liquid phase reductant. Additionally, mixing reductant with air breaks up the reductant in small particles, thus resulting in faster evaporation process Additionally, injecting air into the vaporizer unit prevents lacquering and soot deposits on the surface of the heating element. Further, the inventors have recognized that dynamically controlling the temperature of the heating element to take advantage of the heat supplied by the exhaust gasses prevents overheating, improves the heating element durability and reduces power consumption.
Yet another advantage of the present invention is that the heating element temperature can be controlled to ignite the injected reductant and air mixture, and thus produce carbon monoxide (CO), which further improves NOx reduction in the ALNC.
It is a further advantage of this invention that CO generation is increased (and thus NOx conversion efficiency increased) by placing an oxidizing catalyst in the path of the reductant and air mixture prior to its mixing with the exhaust gasses.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.